As the length of railways increases globally, the amount of freight transportation by rail is increasing year by year. Freight cars such as railway wagons and containers are used for railway freight transportation, and ferritic stainless steel is nowadays used as a material for freight cars.
However, there is a problem in that ferritic stainless steel, which has insufficient low-temperature toughness, is not suitably used in cold areas in, for example, inland regions of the Eurasian Continent having an atmospheric temperature of −30° C. or lower in the winter. In particular, a material for the body of a freight car carrying liquids such as oil is required to have excellent low-temperature toughness.
Moreover, in ferritic stainless steel, there is a problem of further deterioration in the toughness of a welded heat-affected zone due to coarsening of grains. Therefore, in cold areas, it is difficult to use ferritic stainless steel in applications to a structure formed by welding.
As an example of stainless steel to be used for a railway wagon, Japanese Unexamined Patent Application Publication No. 2012-12702 discloses a stainless steel in which the corrosion resistance of a weld zone is improved by forming a martensite phase in a welded heat-affected zone and in which the occurrence of surface defects is suppressed by specifying an FFV value. However, that stainless steel has insufficient low-temperature toughness.
As an example of stainless steel sheet having excellent toughness, for example, Japanese Unexamined Patent Application Publication No. 11-302791 discloses a high-strength high-toughness stainless steel sheet having excellent bendability. In that high-strength high-toughness stainless steel sheet, bendability is improved by controlling the length of MnS-based inclusion particles in the rolling direction to be 3 μm or less and by controlling the ratio of the length in the rolling direction to the length in a direction at a right angle to the rolling direction of the MnS-based inclusion particles to be 3.0 or less. However, in JP '791, corrosion resistance, in particular, the corrosion resistance of a weld zone, which is required for a material for the body of a freight car, may be insufficient and further low-temperature toughness may be insufficient, in some cases.
Japanese Unexamined Patent Application Publication No. 61-136661 discloses a thick martensitic stainless steel having excellent toughness in which formation of δ ferrite is inhibited. However, since the strength of that stainless steel is excessively high, it is difficult to perform press forming on that stainless steel to use it for a railway wagon or a container for railway freight. In addition, in the stainless steel described in JP '661, low-temperature toughness may be insufficient in some cases.
In addition, as an example of a ferritic stainless steel having improved low-temperature toughness of a welded heat-affected zone, Japanese Unexamined Patent Application Publication No. 2003-3242 discloses a ferritic stainless steel having excellent toughness of a welded joint. However, coarsening of grains in a welded heat-affected zone is inhibited by causing fine Mg-based oxides to be dispersed and precipitated in steel.
Japanese Unexamined Patent Application Publication No. 4-224657 discloses a ferritic stainless steel having excellent toughness of a welded heat-affected zone. However, the toughness of a weld zone is improved by adding Co.
However, JP '242 and JP '657 are not sufficient to provide toughness of a welded heat-affected zone to be used in a cold area having an atmospheric temperature of −30° C. or lower.
As described above, the stainless steels described above are not suitable as a material for a freight car carrying liquids such as oil in a cold area because of their insufficient low-temperature toughness. In addition, the stainless steels disclosed above do not have satisfactory corrosion resistance or workability which is required for a material for the body of a freight car.
Moreover, since there is a further deterioration in the low-temperature toughness of a welded heat-affected zone, those stainless steels are not suitably used in applications in which a structure is formed by welding.
It could therefore be helpful to provide a ferrite-martensite dual-phase stainless steel having satisfactory corrosion resistance and workability required for a material for the body of a freight car, and having excellent low-temperature toughness and to provide a method of manufacturing the stainless steel.
In addition, it could also be helpful to provide a ferrite-martensite dual-phase stainless steel to be used as a material for a welded structure excellent in terms of the low-temperature toughness of a welded heat-affected zone in addition to the properties described above and a method of manufacturing the stainless steel.